The present invention relates to a fuel injection valve for use in internal combustion engines, especially diesel engines.
It is required in diesel engines in general to set the fuel injection rate, that is, a fuel amount to be injected per unit time, at high values so as to obtain satisfactory output characteristics of the engine and reduce noxious components such as nitrogen oxides in the exhaust gases. However, an increase in the fuel injection rate is accompanied by a corresponding decrease in the fuel injection period of time, which causes sudden combustion of fuel within combustion chambers, causing combustion noise as well as an excessive increase in the pressure within cylinders.
Conventional fuel injection valves of this kind have the problem that the nozzle needle lifts at the same increasing rate in a small injection quantity region as in a large injection quantity region, which causes unstable injection, i.e. so-called irregular injection. That is, in the small injection quantity region the lifting amount becomes excessive in one cycle of injection to thereby cause an excessive injection quantity, which is followed by an excessive decrease in the fuel injection quantity in the next cycle due to a reduction in the pressure within the injection pipe connecting the fuel injection valve to a fuel injection pump, and then the injection quantity becomes again excessive in the following cycle.
To overcome this disadvantage, a fuel injection valve has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 59-46364, in which a first spring acting at the initial stage of injection and a second spring acting at the following main injection are provided in a nozzle holder, wherein an initial injection stroke is effected under a valve-opening pressure determined by the force of the first spring, while a main injection stroke subsequent to the initial injection stroke is effected under a valve-opening pressure determined by the sum of the forces of the first and second springs, whereby the fuel injection rate is reduced throughout the whole injection stroke, to thereby prevent generation of combustion noise and reduce noxious components in the exhaust gases, as well as prevent irregular injection.
The proposed fuel injection valve is constructed such that a nozzle needle is arranged within a nozzle mounted on an end of a nozzle holder and abuts against a first movable spring seat, and the first spring is interposed between the first movable spring seat and a first stationary spring seat and determines the valve opening pressure of the nozzle needle during the initial injection stroke. Further, a rod having a second movable spring seat mounted thereon is disposed within the nozzle holder, with its end face facing the nozzle movable in spaced and opposed relative to the first movable spring seat, with a gap corresponding to the initial injection life of the nozzle needle therebetween. A second spring is interposed between the second movable spring seat and a second stationary spring seat, and determines the valve opening pressure of the nozzle needle during the main injection stroke.
The proposed fuel injection valve uses five component parts to separately adjust the setting forces of the first and second springs with ease, namely, a supporting member for the second movable spring seat, an externally threaded member for supporting an end of the second spring seated thereon, a screw for adjusting the setting force of the second spring, a lock nut for fixing the adjusting screw in position, and a cap nut for covering the externally threaded member. Thus, numerous component parts are required, which causes an increase in the number of steps of assembly, resulting in an increase in cost. Further, the supporting member for the second movable spring seat is cylindrically shaped, with a whole body thereof fitted in the nozzle holder, the externally threaded member being also cylindrically shaped, with a portion thereof threadedly fitted in a tapped bore formed in an end portion of the nozzle holder remote from the nozzle, the cap nut being also cylindrically shaped and threadedly mounted on the outer peripheral surface of the externally threaded member. Thus, many component parts are coaxially fitted on the nozzle holder, so that the fuel injection valve is large in radial size as a whole, as well as large in weight.